Continuous polarographic analysis for low hydrogen sulfide concentrations



Feb- 14, 1967 L A. cAPUANo CONTINUOUS POLAROGRAPHIC ANALYSIS FOR LOW HYDROGEN SULFIDE CONCENTRATIONS Filed Aug. 8, 1965 O O O O O HU @T n u I Ib@ ATTORNEY United States Patent O 3,304,243 CONTINUUS PLAROGRAPHIC ANALYSIS FOR LOW HYDROGEN SULFDE CONCENTRATIONS Italo A. Capuano, Grange, Conn., assignor to Union Carbide Corporation, a corporation of New York Filed Aug. 8, 1963, Ser. No. 300,821 6 Claims. (Cl. 21M-1) The p-resent invention relates to a continuous polarographic method for the qualitative and quantitative determination of relatively low concentrations of hydrogen sulfide occurring in multi-component mixtures. The invention provides for the detection and measurement of low hydrogen sulfide concentrations in a method which utilizes the reaction product of ammonium hydroxide and a hydrogen sulfide-bearing sample stream as a supporting electrolyte. The present invention is especially useful in gas stream analyses in the chemical manufacturing indus- Hydrogen sulfide is a toxic contaminant being encountered more and more in industrial processes, particularly the gas processing industry, notedly in natural gas streams. Environmental occurrence of hydrogen sulfide in even small amounts occasions danger to human life. The American Conference of Government Hygenists has set a safe threshold exposure limit for humans of 20 parts per million by volume over an eight hour breathing period. Hydrogen sulde, it must be noted, can cause olfactory paralysis in anywhere from 2 to l5 minutes of exposure and render a person so exposed vulnerable to unwittingly breathing greater quantities sufiicient to bring on sickness and death. See 42 Hydrocarbon Processing & Petroleum Refiner 115, April 1963. The present invention provides a method for .the accurate detection and measurement of this toxic contaminant utilizing a polarogr-aphic technique.

The polarograph developed by Heyrovsky and Shikata is now a fairly well-known laboratory instrument which is used to produce a continuous visible indication on a recording chart or the like of a current-voltage curve which is characteristic of a solution undergoing electrolysis between a continually dropping mercury electrode anda reference electrode. See Polarography, 10 Encyclopedia of Chemical Technology 886, The Interscience Encyclopedia Inc., New York. The instrument as such has particular -applicability where the electrolysis action involved would, in time, coat and render ineffectual a stationary type electrode of conducting metal or such. Each new drop of mercury forming at the end of a capillary in a measuring cell portion of a polarograph presents to the electrolysis which is underway a clean, new conducting surface for current passage. Controls are provided on conventional polarographic apparatus to permit the selection of desired voltage ranges and sensitivity ranges to suit whatever recorder or indicating instrument is used. The current-voltage indicia are read out proportional to the ion concentration of whatever component of interest is then undergoing electrolysis between the fixed and the mercury dropping electrodes.

While conventional polarographs have enjoyed useful applications and produced many beneficial results in the laboratory, their application to plant stream analyses has, prior to the time of the present invention, not been successfully accomplished. One of the main deficiencies of known -polarographic apparatus is the requirement that the mercury used in conjunction with dropping mercury measuring electrodes must be gathered at the bottom of the measuring cell, drained out from time to time, cleaned as required and redeposited in some sort `of reservoir subtending the measuring cell portion of the polarograph. Another problem which obtains is that the conventional polarographic mercury drop time of from 2 to 6 seconds is disturbed as to periodicity and drop size by external 3,304,243y Patented Feb. 14, 1967 ICC vibrations encountered in production a-reas. This effects instrument stability and polarograph diffusion current reproducibility.

In my copending application entitled Continuous Polarographic Analyzer I disclose 'an advanced type of polarographic apparatus and method which are particularly adaptable to plant stream uses in commercial production processes. While the invention described in the aforesaid application is usable in polarographic techniques generally, it is particularly useful in the polarographic `analysis for relatively low concentrations of hydrogen sulfide according to the present invention and will therefore be described hereinafter to the extent considered necessary for an understanding of the present invention.

It is therefore an object of this invention to provide a novel method for the continuous detection and measurement of parts per million concentrations of hydrogen sulfide in multicomponent mixtures such as gas streams and the like through the pol'arographic technique.

It is a further object of this invention to provide such an analytical method for applications where the hydrogen sulfide concentration range is from 0 to 100 parts per million.

It is a still further and important 'object of this invention to provide a method for hydrogen sulde analyses according to the polarographic technique which is particularly suitable for plant stream adaptation, i.e. the analyses of continuously iiowing streams encountered in industrial processes and the like.

In general, the analytical method according to my invention comprehends the steps of reacting substantially pure water with anhydrous ammonia to produce la solution of ammonium hydroxide, passing a stream of multicomponent mixture containing a low -hydrogen sulfide `concentration continuously through the ammonium hydroxide solution to produce a solution of ammonium sultide, continuously passing a stream of the ammonium sulfide solution thus produced through a measuring cell Iportion of a polarographic analyzer apparatus, concurrently continually dropping liquid mercury drops from a dropping mercury electrode disposed in the ammonium sulfide solution passing through the measuring cell portion of the polarographic analyzer at a frequency higher than the normal dropping frequency, i.e. 0.1 to 0.4 second as against 2 to 6 seconds, main-taining an applied electrical voltage between the dropping mercury electrode and a reference electrode disposed in a reference cell portion of said polarographic analyzer; developing a continuous signal representative of voltage-diffusion current relationship obtaining between the dropping mercury and the reference electrodes, and translating said signal into a continuous sensible indication proportional to t-he sulfide ion concentration -of the ammonium sulfide solution passing through the measuring cell portion of the polarographic analyzer, said sensible indication being also proportional to `and representative of the concentration of hydrogen sulfide present in the stream `of multicomponent mixture being analyzed.

With the foregoing and other obects in views, which will become more apparent hereinafter, the invention will now be described in .greater particularly and with reference to the appended drawing which is a combination liow diagram and electrical schematic representation of an apparatus arrangement suitable for performing the method according to the present invention, designed and operated to analyze gas streams for 0 to 100 parts per million (ppm.) concentrations of hydrogen sulfide.

Substantially pure water which has been distilled, deionized or the like is supplied continuously to a constant head water reservoir A 13 through conduit 11. From reservoir 13 the water is fed through conduit 15, water metering device 17 and conduit 19 into a reactor vessel 21. Anhydrous ammonia, NH3, is supplied from a supply source 23 through yconduit 25, NH3 pressure reducing valve 27, conduit 29, NH3 metering device 31 and conduits 33 and 35 to reactor vessel 21. Nitrogen N2 is also supplied from a supply source 37 through conduit 39, N2 pressure reducing valve 41, conduit 43, N2 metering device 45 and conduit 35 to reactor vessel 21.

In the reactor vessel 21, the Water and the anhydrous ammonia react to form ammonium hydroxide NH4OH The nitrogen feed to reactor vessel 21 serves to remove any dissolved oxygen and to mix the ammoniacal solution in the vessel.

From `reactor vessel 21 the ammonium hydroxide thus formed is passed through conduit 47 to a liquid-gas separator 49 from which any gas attending the reaction will escape through conduit 51 to a vent 53. The degassed ammonium hydroxide is then directed through conduit 55 to another reactor vessel 57. A gas sample bearing relatively low concentrations of hydrogen sulfide HZS for which analysis is desired is directed from sample connection point 59 through conduit 61, sample pressure reducing valve 63, conduit 65, sample metering device 67 and conduit 69 to second reactor vessel 57.

In second reactor vessel 57, the ammonium hydroxide and whatever hydrogen sulfide present in the sample react to form ammonium sulfide (NH4)2S and Water H2O From second reactor vessel 57 the ammonium sulfide solution thus formed is passed through conduit 71 to a liquid-gas separator 73 from which any unreacted gas will escape through conduit 75 to the vent 53. The degassed ammonium sulfide solution is then directed through conduit 77 to a measuring cell 79 of an H-shaped polarographic cell pair, the second component of said cell pair being a reference cell 81. The ammonium sulfide solution then passes through measuring cell 79 and out through a discharge conduit 83 to a gravity drain 85 which is vented in the conventional manner as for example iat 87.

In addition to the lce-ll pair 79-81 the sample inlet conduit 77 and outlet conduit 83, the polarographic apparatus componentry comprehends the remaining elements showns at the right side of the drawing. A mercury reservoir 89 subtends the cell pair 79-81 and provides a constant head source for the mercury dropping capillary which forms the measuring electrode of the apparatus. Reservoir S9 is :bottom-connected through a mercury conduit 91 to a mercury dropping capillary 93 arranged in measuring `cell 79 and supported by a measuring cell top closure 95. A top closure 97 is also provided on mercury reservoir 89.

Here it may be noted ,that nitrogen from the nitrogen supply source 37 is also provided to purge the atmosphere above the mercury surface in reservoir 89 and the electrolyte solution surface in measuring cell 79 through the conduit 39, nitrogen pressure reducing valve 41, conduit 43, conduit 99, a nitrogen metering device 101, conduit 103 to the reservoir 89 and conduit 105, which bifurcates from conduit 103, to the measuring cell 79. Purge vents 107 and 109 are also provided respectively in mercury reservoir top closure 97 and measuring cell top closure 95 to assure flow continuity of the purging nitrogen.

The design of polarographic measuring cell 79 is such that mercury dropping continually from capillary 93 collects separately from the electrolytic solution passing through the cell and drains continuously through a mercury trap drain 111 at the bottom of the measuring cell, lthrough a mercury drain conduit 113 into mercury collecting tank 115. From the mercury collecting tank, mercury is continuously recirculated through mercury recirculating conduit 117, mercury pump 119 and mercury recirculating conduit 121 through top closure 97 back into the mercury reservoir 89. An overflow means 141 is provided in the reservoir 89 and arranged to drain olf mercury over a preselected level to the collecting tank through a mercury over-How conduit 143 which connects to mercury drain conduit 113. While it is in the collecting tank 115, the mercury is continuously washed of impurities with water from pure Water supply conduit 11, through a purging Water conduit 123 which connects into mercury collectng tank 115 as shown on the drawing. The mercury in collecting tank 115 may be vigorously scrubbed with a magnetic stirrer or the like in the event that the pure Water stream alone is found inadequate to free the used mercury of ammonium hydroxide-ammonium sulfide impurity and to remove any surface films of mercuric sulde which may occur. A mercury purging Water drain conduit 125 is provided to continuously remove the purging water from tank 115 to drain 85, described before.

Also in the illustrated arrangement of apparatus, the polarograph reference cell is provided with a reference electrode 127 which may tbe for instance a customarily used saturated calomel electrode. A conventional polarograph potential source and current detector 129 is larranged to provide the desired potential across the electrodes, respectively the mercury in reservoir 89 through a conductor 131 and the reference electrode 127 through a conductor 133. Current and voltage signals produced during analyses are transmitted to a conventional polarograph voltage current indicator/recorder 135 through conductors 137-139.

Conventional valves and gages used have been illustrated for purposes of clarity but not designate-d numerically since their uses will be readily apparent to persons familiar with the art.

During an analyses a diffusion current proportional to the potential applied across the mercury and saturated calomel electrodes is developed in t-he polarographic cell, detected in device 129 and translated lalong with a voltage signal to indicator recorder 135. The predominant electrode reaction responsible for the diffusion current Vis believed to be The sulfide ion in a strongly basic medium produces a reversible anodic wave due to the oxidation of mercury to mercuric sulfide and the anodic diffusion current is directly proportional to the sulfide ion concentration and thus indicative of the concentration of the hydrogen sulfide present in the sample being analyzed.

Example l In a typical application of the method and apparatus of the present invention and with reference to the illustrative drawing, purified Water at a iiow rate of 26 milliliters per minute from Water reservoir 13 was reacted continuously in reactor vessel 21 with anhydrous ammonia metered at a ow rate of 80 milliliters per minute. A dow of 460 milliliters per minute of pure nitrogen was simultaneously passed into the Water-anhydrous ammonia reactor vessel 21 to remove oxygen and to mix the ammoniacal solution. After degassification in liquid-gas separator 49, the ammonium hydroxide stream was introduced into reactor vessel 57 simultaneously with a controlled sample stream from 59 flowing at a rate of 500 milliliters per minute. The reaction produ-ct, an ammonium sulfide solution, Was passed through liquid-gas separator 73 and then through the polarograph measuring cell 79. In this particular application, a mercury dropping capillary Was fabricated from a 4-inch long piece of thermometer capillary stock with an elliptical bore having a major axis dimension of 0.045 millimeter and a minor `axis dimension of 0.012 millimeter. Mercury reservoir 89 .Was adjusted for a static head on the capillary of about 8 inches to deliver a mercury mass flow rate of 54 milligrams per second at a drop internal of 0.35 second. The

mercury collected in tank 115 was continuously washed with a stream of purified water through purging water conduit 123 vigorously agitated with the collected mercury by means of a magnetic stirrer. The device 129 =was adjusted to apply a fixed potential of -0.6 Volt D.C. to the dropping mercury electrode, referred to a saturated calomel electrode 127. A positive displacement metering pump 119 was used to recirculate the washed mercury tio the reservoir 89. The voltage-current indicia procountered, typical refinery gas ystream constituents and constituent groups were added in their normally occurring concentrations to samples of hydrogen sulfide-nitrogen perpared as in Example II, i.e., 50 parts per million hydrogen sulfide in nitrogen. Operating conditions were the same as in Example Il. The effect of the presence of t-hese other substances on the analyses for hydrogen sulfide concentrations was found to be negligible as can be seen from the data presented in Table II below.

TABLE II.-EE:`6FECT OF VARIOUS GASES ON THE POLAROGRAPHIC ETERMINATION OF HYDROGEN SULFIDE duced was recorded on a commercially available one millivolt input circular chart recorder.

The aforedescribed arrangement of apparatus and operating conditions was used with outstandingly successful results in analyses of 0-10 parts per million of hydrogen sulfide in nitrogen. Accurately prepared samples containing hydrogen sulfide -concentrations within this range Were analyzed continually and the results were highly accurate and consistently reproducible.

Example 1l In a test to evaluate the long term 4stability of the system according to the invention under plant operating conditions an arrangement similar to that of Example I was constructed in an operating plant area adjacent to some continuous production gas separation compressors which operated with relatively high attendant vibration and noise. A sample was prepared |by admixing 50 parts per million by volume of hydrogen sulfide in nitrogen and this sample was continuously passed through the analyzer for a day-long test at a ow rate of 500 milliliters per minute. Water flow rate was 42 milliliters per minute; ammonia fiow rate 80 milliliters per minute; nitrogen flow rate 460 milliliters per minute; and the mercury drop frequency was adjusted to 0.1 second. Table I below presents the results of this experiment, showing the readout value of the concentration at hourly intervals over the test duration, and illustrates the constant accuracy of the analysis. Even as to differences found, it will be appreciated by persons familiar with the art, the reading may well lbe perfectly accurate, the difference from the prepared 50 parts per million sample being attributable to incomplete admixing or adventitious overconcentrations of the hydrogen sulfide in the sample stream nitrogen.

TABLE I.-LONG TERM STABILITY OF THE CONTINUOUS POLARO GRAPHIC ANALYZER 1N HgS ANALYSES Example III In a test to ascertain the effect of the presence in sample streams of other substances which might be en- The preparatory steps required in processing a sample to be analyzed may, in a given instance, be less involved or more involved than those at-tending the analyses described hereinabove. It may be found for example that deoxygenation of the sample is in order since oxygen is known to interfere with hydrogen sulfide determination. Any other-occurring substances which may have an interfering effect on the analysis should also be eliminated or neutralized in a preparatory step or steps. In the polarographic method the concentration and conductance of the supporting electrolyte have a definite influence on the diffusion current. Thus it is important that any materials occurring in the -sample stream which will react to produce ions other than sulfide ions, which other ions are close enough to sulfide ions in polarographic diffusion current response to produce an interfering effect, lbe removed or neutralized physically or chemically lest they interfere with the sought hydrogen sulfide concentration determination. Example III and Table II, above show that the normally occurring constituents in refinery gas streams do not present any interference problems in this respect. In cases where the samples to be analyzed `do present such problems, it is considered well within the skill of the analytical chemist routinely to devise and perform Whatever preparatory corrective steps may `be required.

Experimental work has shown that full scale analyses for hydrogen sulfide concentrations can be performed easily in any desired parts per million range from up to many hundred parts per million to full scale readouts of as low as 10 parts per million and lower by merely varying the sample and reagent fiow rates. Lowering the sample dow rate and/0r increasing the reagent flow rate permits determinations in the higher concentration ranges. Decreasing the reagent flow rate and/or increasing the sample flow rate extends the determination capability to the lower H28 concentration ranges.

The temperature at which .analyses are conducted also affects the polarographic diffusion current. The diffusion current varies approximately 2 percent per degree centigrade change in temperature, necessitating system temperature control. The illustrative experimental work described hereinabove was performed at a maintained temperature of 30 centigrade.

From the foregoing description and examples it will be appreciated that the present invention provides a significant advance in analytical technique for the -determination of hydrogen sulfide, a toxic contaminant being encountered more and more in industry, particularly the gas processing industry. The disclosure and drawing are intended for illustrative purposes only, the invention being defined by the appended claims.

What is claimed is:

1. A method for continuous qualitative and quantitative determination of concentrations of hydrogen sulfide in multicomponent mixtures which method comprises the steps of reacting substantially pure water with anhydrous ammonia to produce a solution of ammonium hydroxide; passing a stream of multicomponent mixture containing hydrogen sulfide continuously through said ammonium hydroxide solution to produce a solution of ammonium sulfide; continuously passing a stream of said ammonium sulfide solution through a measuring cell portion of a polarographic analyzer apparatus; concurrently continually dropping liquid mercury drops from a dropping mercury electrode arranged in said measuring cell portion and disposed in the ammonium sulfide solution passing therethrough; maintaining an applied electric voltage between the dropping mercury electrode and a reference electrode disposed in a reference cell portion of said polarographic analyzer; developing a continuous signal representative of voltage-diffusion current relationship obtaining between the dropping mercury and the reference electrodes; and translating said signal into a continuous sensible indication proportional to the sulfide ion concentration of the ammonium sulfide solution passing through the mercury cell Iportion of the polarographic analyzer apparatus, said sensible indication being also proportional to and representative of the concentration of hydrogen sulfide present in the stream of multicomponent mixture being analyzed.

2. A method -according to claim 1 wherein the step of translating said signal into a continuous sensible indication includes production of a visually sensible indication proportional to and representative of the concentration of hydrogen sulfide present in the stream of multicomponent mixture being analyzed.

3. A method according to claim 1 wherein the step of dropping liquid mercury drops through said measuring cell portion of the polarographic analyzer apparatus is performed with a time interval between discrete drops of from about 0.1 to about 0.4 second.

4. A method for continuously detecting and quantitatively measuring concentrations of hydrogen sulfide in multicomponent essentially gas streams which method comprises the steps of continuously reacting a stream of substantially pure water With a stream of anhydrous ammonia to produce an ammonium hydroxide solution; degasifying the ammonium hydroxide solution produced by the reaction of said substantially pure water with said anhydrous ammonia; continuously reacting said arnmonium hydroxide solution after its degasification with a continuously flowing stream of gas containing hydrogen sulfide to produce an ammonium sulfide solution; degasifying the ammonium sulfide solution thus produced; continuously passing the ammonium sulfide solution through a measuring cell portion of a polarographic analyzer apparatus; continually dropping substantially pure liquid mercury from a dropping mercury electrode arranged in said measuring cell portion; applying an electric voltage between the mercury in said dropping mercury electrode and a reference electrode disposed in a reference cell portion of said polarographic analyzer apparatus; continuously developing a voltage-diffusion current signal proportional to the sulfide ion concentration obtaining in the ammonium sulfide solution passing through said measuring cell portion7 and translating said signal into a continuous sensible indication proportional to the sulfide ion concentration of the ammonium sulfide solution passing through the mercury cell portion of the polarographic analyzer apparatus, said sensible indication being also proportional to and representative of the concentration of hydrogen sulfide present in the stream of multicomponent mixture being analyzed.

5. A method according to claim 4 wherein the step of translating said signal into a continuous sensible indication includes production of a visually sensible indication proportional to and representative of the concentration of hydrogen sulfide present in the stream of multicomponent mixture being analyzed.

6. A method according to claim 4 wherein the step of dropping liquid mercury drops through said measuring cell portion of the polarographic analyzer apparatus is performed with a time interval between discrete drops of from about 0.1 to about 0.4 second.

References Cited by the Examiner Kolthoff et al.: Polarography, 2nd edition, 1952, pages 563 and 564.

JOHN H. MACK, Primary Examiner.

T. H. TUNG, Assistant Examiner. 

1. A METHOD FOR CONTINUOUS QUALITATIVE AND QUANTITATIVE DETERMINATION OF CONCENTRATIONS OF HYDROGEN SULFIDE IN MULTICOMPONENT MIXTURES WHICH METHOD COMPRISES THE STEPS OF REACTING SUBSTANTIALLY PURE WATER WITH ANHYDROUS AMMONIA TO PRODUCE A SOLUTION OF AMMONIUM HYDROXIDE; PASSING A STREAM OF MULTICOMPONENT MIXTURE CONTAINING HYDROGEN SULFIDE CONTINUOUSLY THROUGH SAID AMMONIUM HYDROXIDE SOLUTION TO PRODUCE A SOLUTION OF AMMONIUM SULFIDE; CONTINUOUSLY PASSING A STREAM OF SAID AMMONIUM SULFIDE SOLUTION THROUGH A MEASURING CELL PORTION OF A POLAROGRAPHIC ANALYZER APPARATUS; CONCURRENTLY CONTINUALLY DROPPING LIQUID MERCURY DROPS FROMA DROPPING MERCURY ELECTRODE ARRANGED IN SAID MEASURING CELL PORTION AND DISPOSED IN THE AMMONIUM SULFIDE SOLUTION PASSING THERETHROUGH; MAINTAINING AN APPLIED ELECTRIC VOLTAGE BETWEEN THE DROPPING MERCURY ELECTRODE AND A REFERENCE ELECTRODE DISPOSED IN A REFERENCE CELL PORTION OF SAID POLAROGRAPHIC ANALYZER; DEVELOPING A CONTINUOUS SIGNAL REPRESENTATIVE OF VOLTAGE-DIFFUSION CURRENT RELATIONSHIP OBTAINING BETWEEN THE DROPPING MERCURY AND THE REFERENCE ELECTRODES; AND TRANSLATING SAID SIGNAL INTO A CONTINUOUS SENSIBLE INDICATION PROPORTIONAL TO THE SULFIDE ION CONCENTRATION OF THE AMMONIUM SULFIDE SOLUTION PASSING THROUGH THE MERCURY CELL PORTION OF THE POLAROGRAPHIC ANALYZER APPARATUS, SAID SENSIBLE INDICATIONB EING ALSO PROPORTIONAL TO AND REPRESENTATIVE OF THE CONCENTRATION OF HYDROGEN SULFIDE PRESENT IN THE STREAM OF MULTICOMPONENT MIXTURE BEING ANALYZED. 